Raised artificial leathers such as suede finished artificial leathers and nubuck artificial leathers which have a raised surface made of the fiber bundles on a substrate comprising fiber bundles and an elastic polymer have been known. The raised artificial leathers are required to fully satisfy a high level of physical properties such as fastness to light, pilling resistance and abrasion resistance, in addition to sensuous properties such as appearance (surface feeling closely resembling natural leathers), hand (soft touch combined with a moderate fullness and a dense feeling), and color development (brilliantness and depth of color). To meet such requirements, there have been made various proposals.
To meet the requirement on the appearance and hand, for example, it has been generally employed to make artificial leathers from microfine fibers. In the production of the artificial leathers made of microfine fibers, it has been widely used to convert composite fibers such as sea-island fibers and multi-layered fibers to microfine fiber bundles by splitting or removal of a polymer component by decomposition or extraction. The raised artificial leathers and grain-finished artificial leathers, which are made from a substrate for artificial leather comprising a nonwoven fabric of microfine fiber bundles resulted from the composite fibers and an elastic polymer impregnated into the nonwoven fabric, are rated highly in their appearance and hand. However, such artificial leathers involve a problem of lowering the color development as the fiber fineness is decreased, to cause a remarkable deterioration in the brilliantness and depth of color. Particularly, the raised artificial leathers fail to meet a general requirement for high quality.
The nonwoven fabric body for the substrate for artificial leathers is generally produced by a method which includes a step of cutting spun fibers into staple fibers having a length of 100 mm or less, a step of making the staple fibers into a nonwoven web having a desired mass per unit area by a carding or paper-making method, a step of optionally superposing two or more nonwoven webs, and a step of entangling the fibers by a needle-punching or spun-lacing method. Using the nonwoven fabric body having a desired bulkiness and a degree of entanglement thus produced, the substrate for artificial leathers is produced. The raised artificial leathers and grain-finished artificial leathers produced from such a substrate for artificial leathers are highly rated particularly in their hand. Although the staple fibers constituting the nonwoven fabric body are fixed in the substrate by the entanglement between fibers and the impregnated elastic polymer, the staple fibers on the raised surface of raised artificial leathers or in the interface between the substrate and the grain layer of grain-finished artificial leathers unavoidably tend to be easily pulled out or fallen from the nonwoven fabric body because of their short length. With this tendency, the important surface properties such as the abrasion resistance of raised surface and the bonding/peeling strength of grain layer are reduced. To remove this problem, there have been generally employed to increase the degree of entanglement, bond the fibers with each other, or impregnate an elastic polymer in a large amount so as to strongly bind the fibers. However, the increase in the degree of entanglement and the use of an increased amount of elastic polymer in turn remarkably deteriorate the hand of artificial leathers. Thus, it is difficult to satisfy the requirements for the appearance, hand and surface properties simultaneously.
To improve the surface abrasion resistance of raised artificial leathers, typically the pilling resistance of raised fibers, there has been proposed to produce suede-finished artificial leathers by a method including a step of making a nonwoven fabric from sea-island fibers which are capable of being converted into bundles of microfine fibers of 0.8 D or less; a step of entangling the nonwoven fabric by needle punching; a step of immersing the entangled nonwoven fabric in an aqueous solution of polyvinyl alcohol (PVA) and then drying it to temporally fix the shape of the nonwoven fabric; a step of removing the sea component from the sea-island fibers by extraction using an organic solvent; a step of impregnating a solution of polyurethane in dimethylformamide (DMF) and coagulating the polyurethane; and a step of raising the surface (Patent Document 1). It is also proposed to add coarse particles to the microfine fibers, the coarse particles having a particle size lager than a quarter of the fiber diameter and being inert to the fibers.
In Patent Document 2, it is proposed to produce suede-finished artificial leathers by entangling a nonwoven fabric of sea-island fibers by needle punching; impregnating a solution of polyurethane in DMF into the entangled nonwoven fabric and coagulating the polyurethane; removing the sea component by extraction to obtain a leather-like substrate; and raising the obtained leather-like substrate. The fiber bundles constituting the substrate comprise fine fibers A of 0.02 to 0.2 D and microfine fibers B having a fineness of not more than ⅕ of the average fineness of the fine fibers A and less than 0.02 D. The ratio of the numbers of fibers (A/B) in fiber bundles is 2/1 to 2/3. The inside of fiber bundles is substantially free from an elastic polymer. The ratio of the number of fine fibers A and the number of the microfine fibers B (A/B) in the raised fibers is 3/1 or more.
There has been further proposed a method of improving the pilling resistance of suede-finished artificial leathers, in which the foot of raised fibers is anchored by partially dissolving the elastic polymer around the foot of raised fibers using a solvent (Patent Document 3).
Patent Document 4 proposes a method of producing a long-fiber nonwoven fabric which is capable of being converted into nubuck artificial leathers having a surface touch with fine texture. In the proposed method, the strain, which is characteristic of a long-fiber nonwoven fabric and caused during the entangling treatment, is relieved by intentionally cutting the long fibers during the entangling treatment by needle punching, thereby exposing the cut ends of fibers to the surface of nonwoven fabric in a density of 5 to 100 μmm2. It is also proposed to regulate the number of fiber bundles within 5 to 70 per 1 cm width on the cross section parallel to the thickness direction of nonwoven fabric, i.e., regulate the number of fiber bundles which are oriented by needle punching toward the thickness direction within 5 to 70 per 1 cm width. It is further proposed to regulate the total area of fiber bundles on a cross section perpendicular to the thickness direction of nonwoven fabric within 5 to 70% of the cross-sectional area.
Patent Document 5 proposes an entangled nonwoven fabric made of long fibers which are capable of being converted into microfine fibers of 0.5 D or less, in which the percentage crimp of long fibers is 10% or less and the nonwoven fabric contains the fibers in a density of 0.25 to 0.50 g/cm3.
In the method of Patent Document 1, since the solution of polyurethane in DMF is impregnated and coagulated after removing the sea component of the sea-island fibers by extraction, the polyurethane penetrates into the inside of microfine fiber bundles, thereby making the hand hard. In addition, a soft hand and touch are not obtained because the coarse particles are added to the fibers.
In the method of Patent Document 2, since the solution of polyurethane in DMF is impregnated and coagulated before removing the sea component of sea-island fibers by extraction, the microfine fiber bundles are substantially free from the polyurethane on their outer surface and in their inside. Therefore, a soft hand and touch are obtained. However, since the microfine fiber bundles are not fixed together by polyurethane, the pilling resistance is insufficient.
Patent Document 3 merely teaches to anchor the foot of raised fibers by partially dissolving the elastic polymer on the outermost surface of the leather-like substrate. Therefore, the fibers in the leather-like substrate are less fixed and the elastic polymer holds the fibers weakly. Therefore, the proposed method is not effective for improving the pilling resistance when the fineness is 0.01 D or more.
In the method of Patent Document 4 for obtaining the long-fiber nonwoven fabric body, the long fibers are cut while preventing the properties from being made lower than intended. However, since a large number of long fibers are actually cut, the advantages of long fibers that the strength of nonwoven fabric is enhanced because of their continuity are significantly reduced, thereby failing to effectively use their advantages. In Patent Document 4, the entangling treatment is not employed for entangling the long fibers from the surface of long-fiber nonwoven fabric, through the inside thereof, to the opposite surface, but employed for cutting the fibers on the surface of nonwoven fabric evenly to produce an extremely large number of cut ends as many as 5 to 100/mm2. Therefore, the entangling treatment should be performed by needle punching under conditions far severer than generally used. In addition, since the fibers to be entangled for the production of the long-fiber nonwoven fabric body are, like known staple fibers, extremely thick fibers of 2.8 D or more, the long fibers cannot be entangled and compacted sufficiently, thereby failing to obtain high-grade nubuck artificial leathers aimed in the present invention.
Although the method of Patent Document 5 improves the denseness, a substrate for artificial leather impregnated with an elastic polymer having a soft hand cannot be obtained because of a high existence density of fibers.
[Patent Document 1] JP 53-34903A (pages 3 and 4)
[Patent Document 2] JP 7-173778A (pages 1 and 2)
[Patent Document 3] JP 57-154468A (pages 1 and 2)
[Patent Document 4] JP 2000-273769A (pages 3 to 5)
[Patent Document 5] JP 11-200219A (pages 2 and 3)